Electronics Manufacturing

The Role of Impedance Control in Electronic Circuits

Impedance Control Test

In electronics, input impedance and output impedance play a crucial role. Impedance control is very important in electronic circuits and devices. Also, we can define impedance in an electronic circuit where it needs to be controlled.

What is Impedance Control

To understand impedance control, we need to know the similar terms, resistance and reactance first. Resistance and reactance have the same unit, and that is ohm. So guess, what is the difference between them? Most people are confused about this. So here see the difference between them.

What is Resistance

It is like a breaker in the road, So because of the breaker every vehicle. Resistance is present in all electrical components and materials and causes a drop in voltage across the component when current flows through it. Because of resistance power dissipates in the form of heat. When the flow of charge goes in the conductor then energy is dissipated as heat, and this is a common characteristic of resistors. Resistance usually says “R” in the circuit and it is measured in ohms.

What is Reactance

It opposes the change in current. Reactance occurs in capacitors and inductors and is generally seen in AC so that is why here we calculate through frequency. Inductive reactance stops changes occurring in current, instead of capacitive reactance stops changes occurring in voltage. Reactance does not cause heat, so there is no power dissipation problem. Reactance does not result in the dissipation of power as heat.

Reactance is denoted by the symbol “X” and is also measured in ohms (Ω). Reactance happens in both ways positive as well negative because of alternating current. The reactance graph shows as resistance resist, reactance resist the flow of AC. Capacitive reactance always decreases when it works in AC or inductive reactance increases in the same situation.

There are two types of reactance: Capacitive Reactance and Inductive Reactance.

  1. Capacitive Reactance: Capacitive reactance will decrease as the input frequency increases, due to this, frequency and ohm graphs go down, and in the graph, it is a downward sloping line.
  2. Inductive Reactance: Inductive reactance will increase as the input frequency increases, Due to this, frequency and ohm graphs go up and in the graph, it is upward sloping line.

To understand more easily, let’s take an example: There is a circuit which has one amplifier with input and output. Input impedance is connected parallel because it connects through the ground. And output impedance connects in series. This circuit comes inside the amplifier.

Circuit has one amplifier with input and output

Input impedance is represented as Zin. Input Impedance is the combined effect of all resistance, capacitance and inductance connected input circuit of the device. As we know capacitance has capacitive reactance and inductor has inductive reactance. It is affecting frequency. In particular, care for impedance control more in high-speed signals.

Usually, the input impedance should be high, like ten times the output impedance of the circuit. Like Input impedance, Output impedance is the combined effect of all resistance, capacitance and inductance connected input circuit of the device. Usually, output impedance should be low. Output Impedance is represented as Zout.

Also read: Electronics Product Development from Idea to Finished Device

Understand Impedance Matching

If Input impedance (source) = Output impedance (load). Then the signal will have no loss.

Let’s, calculate Impedance through capacitive load:-

Capacitive reactance:- Xc=1/ωC=1/2πfC

So now to understand it make it easy:

Impedance, Z = √(R2 + X2) where X = XL – XC, As we can see, we are talking about capacitive reactance So X= – XC.

We can put this value in the Impedance formula to calculate it.

Similarly, if we want to calculate Impedance through Inductive reactance:-

Inductive reactance:- XL = 2πfL

Impedance, Z = √ (R2 + X2)) where X = XL – XC, Now we are talking about inductive reactance So X= XL. Accordingly, we can calculate impedance.

If we have both reactance then we can calculate impedance and get impedance controlled.

How to Measure the Input Impedance

Calculate impedance in input and output

Let’s see how to calculate impedance in input and output through the signal frequency. In the lab, the requirement of the signal generator, an oscilloscope and a variable resistor. Variable resistor connects in series with input pin (Input impedance) Zin of the amplifier. Output impedance (Zout) connects in the oscilloscope through the load. Here we can see the noise in the input signal by making some changes in signal amplitude.

Do some freq. change from the signal generator. We can see the output on the Oscilloscope or AC volt meter.

Measurement for output impedance the only change is that the variable resistor is placed after Zout of amplifier.

Also read: Importance of Multilayer PCB Fabrication in Electronics Manufacturing

Why Impedance Control is Important in PCB Electronic Circuit

Impedance control in PCB design means the signal travels smoothly without any distortion or noise. This is important, especially when dealing with fast signals, to avoid issues like signal distortion and interference. Impedance matching is necessary in x signals it is like all interconnects in signal from transmitter to receiver to have the same impedance. The characteristic impedance of the transmission line should match the impedance of the source and load to minimize signal reflections. In high-speed design, there is a transmission line which means trace travel with high frequency. These transmission lines have the specific characteristic impedance determined by their physical dimensions, such as width, thickness, and separation.

Impedance Control PCBs

Usually, the impedance of PCB is 50ohm, which is not a high-speed board. As frequency in board increases due to the use of high-speed interfaces like memories, mipi DSI, USB and Ethernet, In that case, use differential pair to maintain 90ohm, 100ohm etc. Here make sure that the PCB stack up keeps accordingly then every high-speed signal gets an equal source and sink. Save signals from crosstalk and maintain signal impedance with tolerance. While matching the impedance there is a need to do signal tunes with very care.

In the case of RF peripherals or sensors, impedance control matching becomes critical for optimal signal transfer and minimal signal loss.
If we do not match impedance control some signal integrity problems can occur, like signal energy may be reflected from load to source in the signal. This can increase the chance of signal distortion, and loss of power.

So, while we define the layer stack up in any PCB layout tool, we can add impedance value at the same time. In schematic design, we must define the net class of impedance of essential diff signal. At the time of routing make sure the routing guidelines. Match all impedance. Make a short trace length. Use the width of the trace as the calculated impedance.

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